The present invention relates to a deviation measurement antenna for a single-pulse radar. It can be applied notably to antennas with a function of providing elements for the measurement of angles that are smaller than the aperture of the antenna beam, these angles defining a divergence between the direction of a detected target and the direction aimed at by the antenna.
Divergence measurement consists of the measurement of the above-mentioned angles in order to locate a target in space with precision. Several divergence measurements are generally used, notably in relative bearing and in elevation. To obtain a divergence measurement for a detected signal, it must be related, by a one-to-one relationship, with an angular divergence from the direction aimed at by the antenna. For this purpose, it is necessary to set up a function of the angular divergence in the interval of measurement, this function being a monotonic one. It is generally an increasing, odd-parity value. In the case of single-pulse radars, where all the information elements are obtained after a single transmission pulse, with this first function associated with a first reception circuit generally called a difference channel or divergence measurement circuit, there is combined a second even-parity function associated with a second reception circuit generally called a sum channel. By standardizing the signal given by the first, monotonic function, this second function can be used to obtain a characteristic signal of the angular divergence of the targets detected by a one-to-one relationship.
Known antennas can be used to set up reception channels needed for measurements of divergence in single-pulse radars. This is the case notably with reflector antennas or passive lens antennas and antennas in arrays with or without electronic scanning.
Reflector or passive lens type antennas have a fixed direction of aim. They are oriented mechanically as required, generally after the processing of the information elements given by the divergence measurement. To form the reception channels, it is possible, for example, to use two horns located on either side of the focal point of the antenna. The two channels are obtained by taking the sum and the difference of the signals coming from the horns, by using for example the orthogonal outputs of a magic T. It is also possible to use a multiple mode horn.
One drawback of this approach arises notably out of the fact that it is difficult to exploit the antenna patterns obtained without an improvement that would increase the complexity of the antenna and hence its cost.
The antenna arrays with or without electronic scanning are used notably to make the radiating apertures of slabs and lenses.
In antennas constituted by arrays of slabs, the signals coming from the radiating elements, distributed on a plane for example, are collected individually and then assembled by combiners that form the necessary channels. The combiners are arranged, for example, as an espalier or a Blass matrix, made in the form of a strip line waveguide or a microstrip line waveguide. These combiners with several channels are fairly complex. The making of these combiners so as to obtain the qualities required by modern antennas is a delicate and costly undertaking.
In the case of lens arrays, the signals coming from the radiating elements, distributed on a plane for example, pass through fixed or electronic phase-shifters and are then radiated to sensors located in the vicinity of a focal point. The phase-shifters are given the task of converting the received wave which is locally plane into a spherical wave centered on the focal point. After this conversion of the received wave, it is possible to use the techniques reserved for reflector antennas, whether or not they are with multiple-mode horns. However, an approach such as this is still complicated and costly to implement. Furthermore, it is cumbersome because of the thickness of the lenses that it causes, notably in the case of an electronic scanning antenna where every element used for the electronic scanning must have this thickness.
The use of digitally controlled precision manufacturing means enables the low-cost manufacture notably of slotted waveguides with very good performance characteristics. However, in an antenna made with slotted waveguides, the weighting of the aperture is achieved by the judicious coupling of the slots to the waveguide. This coupling is fixed once and for all at the time of manufacture and it is difficult to implant a second channel, a second orthogonal weighting to create the even-parity and odd-parity reception channels needed to obtain the divergence measurement.